1. Field of the Invention
The present invention relates to a liquid crystal display device, in particular, to an in plane switching liquid crystal display device having a structure called fringe field switching (FFS).
2. Description of the Related Art
A cross section of a pixel structure of an in plane switching liquid crystal display device having a structure called fringe field switching (FFS) is illustrated in FIG. 4. FIG. 4 is depicted for comparison with FIG. 1 which illustrates an embodiment of the present invention. Therefore, for a detailed structure of FIG. 4 other than the parts described below, see the description given for FIG. 1.
As illustrated in FIG. 4, a thin film transistor TFT, pixel electrodes PD, and a common electrode CD are provided on a surface of a substrate GLS1 on a liquid crystal LC side. The thin film transistor TFT is turned ON by supplying a scanning signal to a gate bus line GL. Through the turned-ON thin film transistor TFT, a video signal is supplied from a data bus line DR to the pixel electrodes PD. The common electrode CD generates an electric field with the pixel electrodes PD. A reference signal serving as a reference for the video signal is supplied to the common electrode CD. The thin film transistor TFT is covered with a passivation PAS formed of a laminate obtained by sequentially laminating an inorganic passivation film PS and an organic passivation film OP so as to avoid direct contact with the liquid crystal LC. On a surface of the organic passivation film OP, for example, the sheet-like common electrode CD made of indium tin oxide (ITO) is formed. The common electrode CD is covered with a capacitor insulator CI. On an upper surface of the capacitor insulator CI, a plurality of the linear pixel electrodes PD arranged in parallel, which are made of, for example, ITO, are formed so as to superimpose the common electrode CD. The pixel electrodes PD are connected to a source ST of the thin film transistor TFT through a through hole TH formed through the organic passivation film OP and the inorganic passivation film PS. In this case, a hole, which is approximately coaxial with the through hole TH and has larger size than that of the through hole TH, is previously formed through the common electrode CD. In this manner, an electrical short-circuit with the pixel electrodes PD is avoided. On a surface of the common electrode CD, on which the pixel electrodes PD are formed, an alignment layer AF1 is formed so as to cover the pixel electrodes PD. The alignment layer AF1 is held in contact with the liquid crystal LC to serve as a layer for determining an initial orientation direction of molecules of the liquid crystal LC.
The liquid crystal display device configured accordingly is driven in the following manner. A voltage is generated between the pixel electrodes PD and the common electrode CD according to the video signal supplied to the pixel electrodes PD. By an electric field generated by the generated voltage, the molecules of the liquid crystal LC are rotated in a plane parallel to the substrate GLS1. This is the reason why the above-mentioned liquid crystal display device is called the in plane switching liquid crystal display device and the above-mentioned structure of the pixel electrodes PD and the common electrode CD is called the fringe field switching (FFS) structure.
FIG. 4 illustrates an electric flux line EF generated by the electric field between the pixel electrodes PD and the common electrode CD.
As a known document relevant to the present invention, for example, JP 2007-183299 discloses a liquid crystal display device. JP 2007-183299A discloses that portions of the capacitor insulator CI between the adjacent pixel electrodes PD are removed until the common electrode CD lying therebelow in the structure illustrated in FIG. 4 is exposed.